1. Field of the Invention
The present invention relates to a process for manufacturing a self-extinguishing cable, such as a cable for low-voltage, medium-voltage or high-voltage power transmission or distribution, as well as a cable for data transmission or for telecommunications, for example a telephone cable, or mixed electro-optical cables.
More particularly, the present invention relates to a process for manufacturing a self-extinguishing cable which has, in a position radially external to at least one transmissive element, at least one flame-retardant coating layer comprising an expanded flame-retardant polymeric material, which is provided with both good mechanical and flame-retardant properties.
2. Description of the Related Art
A self-extinguishing cable is generally produced by providing the cable with a flame-retardant coating layer obtained from a polymeric material, for example a polymeric material based on polyolefin (such as polyethylene or ethylene/vinyl acetate copolymers), that has been given flame-resistance properties by means of suitable additives.
It is well known in the art that to get an effective flame-retardant action in order to minimize if not to avoid the flame propagation, inorganic flame-retardant fillers have to be added in very large amounts to the polymeric material in order to get a polymeric material suitable to realize self-extinguishing cables.
Inorganic flame-retardant fillers free from halogens having flame retardant properties, such as metal hydrated oxides or hydroxides, particularly aluminium or magnesium hydroxide, are among the most widely used fillers. At high temperatures, these products undergo an endothermic decomposition process that generates water, thus depriving the substrate from heat and, therefore, slowing the polymer pyrolysis reactions.
For example, Japanese Patent Application JP 2000/106041 discloses a flexible non-halogen flame-retardant electric cable. Said cable comprises (A) two strands of polyvinylchloride or polyolefin-insulated conductors or multiple strands of these insulated conductors with spacing between the twisted strands and (B) foamed non halogen fire-retardant sheaths which are formed by compounding in the olefin-based resin at least one flame-retardant agent, such as magnesium hydroxide, calcium hydroxide, barium hydroxide, aluminium hydroxide, in an amount of 50-200 wt. parts per 100 wt. parts of the resin, and an organic foaming agent, such as azodicarbonamide, p-toluenesulfonyl hydrazide, or 4,4′-oxybis (benzenesulfonylhydrazide), in an amount of 0.2-5 wt. parts per 100 wt. parts of the resin, possibly adding other processing aids or an antioxidant and getting a foaming rate of 5% to 20%. The abovementioned flame-retardant sheaths are said to have the prescribed tear load and the prescribed bending load so that the tear characteristics, the handling and processability are improved and the standard flame-resistance is ensured. No mention about the process used to make said foamed fire-retardant sheaths is made.
However, the use of these inorganic flame-retardant fillers has a number of drawbacks, the main of which is the fact that, as already reported above, in order to obtain an efficient flame-retardant action, very large amounts of the inorganic flame-retardant fillers need to be added to the polymeric material, in general about 120-250 parts by weight relative to 100 parts by weight of the polymer base. Such large amounts of fillers lead to a decline in the processability and in the mechanical and elastic properties of the resulting flame-retardant composition, in particular, with regard to its elongation at break and its stress at break.
Moreover, on the basis of the Applicant's experience, the presence of such large amounts of inorganic flame-retardant fillers makes extremely difficult to homogeneously expand the resulting flame-retardant polymeric material. As a matter of fact, the obtained expanded flame-retardant polymeric material shows poor appearance, mainly due to the formation of irregular bubbles (i.e. bubbles having irregular form, size and distribution) and swellings, which impair not only its appearance and smoothness but also its mechanical properties.
Processes useful to expand polymeric materials enclosing inorganic fillers are known in the art.
For example, European Patent Application EP 860,465 discloses a process for preparing a foamed thermoplastic article comprising heating and mixing, possibly with a static mixer, a thermoplastic elastomer selected from the group comprising styrene based thermoplastic elastomers and thermoplastic polyolefin elastomers, with an effective amount of a water containing compound, preferably aluminium trihydrate or magnesium hydroxide, to a temperature at which the compound releases water, which is a temperature higher than the melting point of the elastomer, subsequently releasing the resulting heated mixture to atmospheric conditions. The abovementioned process is said to give foamed articles having fine and uniform cell structures.
However, as the water containing compound releases water during the foaming process (i.e. undergoes decomposition during the foaming process), said compound will not be able to confer flame-retardant properties to the obtained foamed articles.